A device for transporting particles of a magnetic material in a selected direction is described in International publication WO 02/34653. The known device contains a cylindrical magnet concentrically arranged within a support member formed by a cylindrical sleeve, of which sleeve the outer surface forms a support surface for supporting the particles. The cylindrical magnet is formed of three smaller magnets stacked together in the axial direction of the separator magnet. Each of the smaller magnets has diametrically opposed N and S poles, and the smaller magnets are stacked in a manner that adjacent magnets have oppositely oriented N-S directions.
The magnet is at its outer surface provided with a number of helical grooves, in order to locally increase the radial distance between the magnet and the support surface to cause regions of lower magnetic field strength. Between these regions of lower magnetic field there are bands of higher magnetic field. The magnet has a central longitudinal axis about which the magnet is rotatable relative to the sleeve. In operation, magnetic particles are retained on the support surface by the magnetic field generated by the cylindrical separator magnet. In particular, the magnetic particles are retained in the regions between the helical grooves where the magnetic field is relatively stronger. Due to axial rotation of the separator magnet, the magnetic particles experience a moving gradient of magnetic field strength perpendicular to the helical groove, which the particles will follow. In this way the particles are transported over the support surface.
It has been found that there is a limit to the number of particles per time unit that the known device is able to transport for a given rotation frequency. If the number of particles that is offered to the device exceeds the limit, the transport of the magnetic particles along the support surface has been found to decline or even stop completely.